Nitrided tantalum base alloys

ABSTRACT

A NOVEL GROUP OF NITRIDED ALLOYS HAVING EXCELLENT WEAR AND ABRASION RESISTANCE CONTAINING AS THEIR MAJOR CONSTITUENT ONE METAL OF THE GROUP COLUMUBIUM, TANTALUM AND VANADIUM ALLOYED WITH TITANIUM AND/OR ZIRCONIUM. SUCH ALLOYS ARE FABRICABLE TO SHAPE AND ARE THEN NITRIDED TO PRODUCE HIGH SURFACE HARDNESS THEREON. THE NITRIDED ALLOYS HAVING UTILITY AS CUTTING TOOL MATERIALS AND OTHER AREAS OF USE WHERE EXCELLENT WEAR AND ABRASION RESISTANCE IS DESIRED.

United States Patent O 3,713,906 NITRIDED TANTALUM BASE ALLOYS Ray J. Van Thyne, Oak Lawn, and John J. Rausch,

Antioch, 11]., assignors to Surface Technology Corporation, Stone Paris, ill. No Drawing. Filed Mar. 4, 1970, Ser. No. 16,571 Int. Cl. C22c 27/00; C23c 11/14 US. Cl. 14831.5 1 Claim ABSTRACT OF THE DISCLOSURE A novel group of nitrided alloys having excellent wear and abrasion resistance containing as their major constituent one metal of the group columubium, tantalum and vanadium alloyed with titanium and/ or zirconium. Such alloys are fabricable to shape and are then nitrided to produce high surface hardness thereon. The nitrided alloys having utility as cutting tool materials and other areas of use where excellent wear and abrasion resistance is desired.

BACKGROUND OF THE INVENTION Our invention relates to a novel group of nitrided binary or ternary alloys consisting of a principal or major constituent one metal of the group columbium, tantalum and vanadium alloyed with titanium and/or zirconium in amounts of percentages by weight as is hereinafter set forth. We have found that such nitrided alloys demonstrate high surface hardness without accompanying brittleness and offer promise for applications requiring wear and abrasion resistance. We would note that the present application is directed to the use of only one metal of the group columbium, tantalum and vanadium in combination with titanium and/ or zirconium.

It is well known that titanium can be nitrided to form a hard surface layer thereon but such material shows a chipping propensity due to brittleness. In the practice of our invention, such brittleness is avoided by specific alloying as taught herein prior to nitriding. As shown subsequently herein, the alloying elements present in typically commercially available titanium alloys do not produce the same improvement and nitrided commercial titanium alloys show chipping similar to nitrided titanium.

The nitriding of titanium-rich alloys, i.e. containing about 90% titanium has been studied previously (for example, see E. Mitchell and P. I. Brotherton, I. Institute of Metals, vol. 93 (1964), p. 381). Others have investigated the nitriding of hafnium-base alloys (F. Holtz, et al., US. Air Force Report IR-7l87 il) (1967); molybdenum alloys (US. Pat. 3,161,949); and tungsten alloys (D. J. Iden and L. Himmel, Acta Met, vol. 17 (1969), p. 1483). The treatment of tantalum and certain unspecified tantalum base alloys with air or nitrogen or oxygen is disclosed in U.S. Pat. 2,170,844 and the nitriding of columbium is discussed in the paper by R. P. Elliott and S. Kornjathy, AIME Metallurgical Society Conference, vol. 10 (1961), p. 367.

In our copending patent applications Wear Resistant Materials Ser. No. 755,658 now US. Pat. 3,549,427 and Wear and Abrasion Resistant Materials Ser. No. 755,662 now US. Pat. 3,549,429, we have disclosed and claimed certain nitrided three through seven metal alloy systems which are characterized by excellent cutting performance. Counterparts to said US. applications have now been issued as Belgium Patents 720,398 and 720,399. Such applications and Belgium patents are directed to nitrided alloys containing (a) one or more metals of the group columbium, tantalum and vanadium;

(b) one or both of the metals molybdenum and tungsten;

and

(c) titanium and/or zirconium,

in certain percentages by weight as is therein set forth.

We have now discovered that certain nitrided binary alloys such as Cb-Ti offer promise for uses requiring abrasion resistance. Similar to the CbTi-W alloys of our copending applications the Cb/ Ti ratio must be above one. Although the over-all performance of the nitrided binary alloys is somewhat lessened compared to the ternary alloys containing tungsten or molybdenum in terms of cutting properties, the present nitrided binary alloys find utility because, for example:

( 1) the unnitrided binary alloys hereof have somewhat improved cold fabricability which is useful for the preparation of the most intricate parts prior to nitriding (although it should be recognized that certain of the unnitrided ternary alloys also possess good cold forming characteristics), and

(2) the use of surface alloying techniques is simplifled-for example, the desired article can be fabricated from readily available columbium which may be titanized to produce the desired Cb-Ti alloy on the surface and then nitrided.

Accordingly, a principal object of our invention is to provide novel nitrided alloys containing one metal of the group columbium, tantalum, and vanadium with titanium and/or zirconium having excellent wear and abrasion resistance and utility as cutting tool materials.

This and other objects, features and advantages of our invention will become apparent to those skilled in this art from the following detailed discussion thereof.

In order to best understand our invention reference should first be had to the experimental procedures which we employed.

EXPERIMENTAL PROCEDURES In our experimental work a series of alloys were melted under an inert atmosphere in a non-consumable electrode arc furnace using a water-cooled, copper hearth. High purity materials (greater than 99.5%) were used for the alloy charges and generally weighed about 50 grams. These procedures are of course quite well known to those skilled in the art.

The alloys were cut into specimens approximately inch thick and reacted in nitrogen at atmospheric pressure. The resulting thickness and microhardnesses of the various reaction zones or layers were determined using standard metallographic techniques. Several tests were used to evaluate the strength and toughness of these materials for potential use in abrasive wear or metal cutting applications.

The metal cutting tests were performed using the nitrided materials as tool inserts x x 42 inch having a 0.030 inch nose radius which was used as a section of the cutting surface. Such radii were ground on the specimens prior to nitriding.

The alloy samples thus prepared were subsequently nitrided. For nitriding we used a cold wall furnace employing a molybdenum heating element and radiation shields with the furnace being evacuated to five microns pressure and flushed with nitrogen prior to heating. Temperatures were measured with an optical pyrometer, namely a Leeds and Northrop Optical Pyrometer, catalog number 862, sighting on an unnitrided molybdenum heating element which completely surrounded the specimens. The temperatures given herein are corrected from this source. We used a correction factor determined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the aforesaid optical pyrometer.

Following nitrided sample preparation, lathe turning tests were run on AISI 4340 steel having a hardness of Rockwell C (Rc), 44. A feed rate of 0.005 inch per revolution and depth of cut of 0.050 inch were used. A standard negative rake tool holder Was employed with a 5 back rake and a side cutting edge angle.

Our principal criterion in determining whether the pres ent nitrided materials passed or failed and thus whether they are useful or not for purposes of the present invention was the ability to remove two cubic inches of hardened steel at a speed of 400 s.f.m. (surface feet per minute).

Furthermore, we evaluated the toughness and chippingresistance by using conical diamond hardness indentation (standard Rockwell A scale-60 kg. load) and evaluating whether chipping occurred around the hardness impression using a 10x eyepiece magnifier. All of our present materials which fall within the scope hereof pass the 400 s.f.m. cutting test and do not exhibit any significant chipping around the Ra hardness indentations. It will be understood of course that our nitrided materials have substantial applicability in Wear and abrasion applications other than cutting, but the cutting test is a standardized and readily reproducible test.

DESCRIPTION OF THE INVENTION We have discovered a novel group of nitrided alloys characterized by high surface hardness and useful wear resistance. These materials are formed when alloys Within our prescribed compositional ranges as hereinafter taught are reacted with nitrogen or an environment which is ni triding to the alloys at elevated temperatures. The hardening reaction is typical of the internal oxidation or nitridation techniques well known in the art. The volume of hard constituents formed at the surface is high in our materials. As nitrogen diffuses inwardly, there is developed a variety of nitrided phases. Nitride formation lessens inward from the surface and this grading contributes to the excellent thermal and mechanical shock resistance of the material. Our materials are characterized by being graded with the degree of nitride formation lessening as one moves inward from the surface.

The alloys of the present invention, in percentages by weight, contain a minimum of 12% of titanium or zirconium or mixtures thereof. The balance, except for minor impurities, consists of either colnmbium or tantalum or vanadium. With either colnmbium or vanadium, the maximum amount of titanium and/0r zirconium is With tantalum the maximum amount of titanium and/ or zirconium is Thus the present invention is directed to nitrided binary or ternary alloys of the following range:

12-35 (T i,Zr) balance Cb, V and 12-40 (Ti,Zr)balance Ta We have also found that in most embodiments hereof the nitrogen weight pick up is at least one milligram per square centimeter of surface area athough nitriding to at least 0.1 mg./cm. is also useful in very thin materials.

As part of the work which led to the present invention unalloyed titanium was nitrided for two (2) hours at 2250 and 2850 F. Both specimens showed chipping at the edges as a result of cooling from the nitriding temperature and Ra hardness indentations resulted in serious chipping all around the impressions. These materials fail immediately in the 400 s.f.m. cutting test.

The possibility that the alloying elements present in commercial titanium alloys might result in an improved nitrided material was examined. Two of the more highly alloyed commercial materials, Ti-6A1-4V and Ti-3Al-l3V- llCr were nitrided at 2850 F. for two (2) hours. Chipping of the edges occurred during cooling and metallographic examination confirmed that similar to unalloyed titanium, thick continuous nitrided surface layers existed.

The Ti-6Al-4V alloy exhibited a 10 mil layer and the Ti-3Al-l3V-11Cr alloy showed a 2 to 5 mil layer with a serrated boundary between the nitrided layer and the substrate. Both of these materials failed immediately in cutting at 400 s.f.m. and are obviously not included within our invention.

Data for cutting tests and chipping resulting from Ra hardness indentions are given in Table I for a wide range of alloys. All of this could be written as a series of examples (and should be considered as such) but for the purpose of brevity We present the data in tabular form. Materials that fall within our invention and pass our test criteria are nitrided alloys consisting essentially of one metal of the group colnmbium, tantalum, or vanadium and titanium and/or zirconium whertin the range of titanium and/or zirconium is 12 to 35% (by weight) when colnmbium or vanadium are present and 12 to 40% when tantalum is present.

TABLE I Nitriding treatment F. Iir. F.

Composition Cutting Chi in (weight percent) st (Rag g Hr. tc

Uualloyed Cb 3,250 Ola-6T1 3,2

OO 22G Unalloycd Ta Ta-5 'I Ta-70 Ti- Unalloyed V Ti 22 OZ 0222220 0 O2 22 OOOOZZZOOQO OOOZZZOO V- Zr": 2, 000 2+ (lb-15 Tins Zr a, 050 2 P Ta-15 Ti-15 Zr 3, 050 2 P P=Pass, removes 2 cu. in. of 43-10 steel workpiece at 400 s.f.m. F=Fails at 400 s.f.m.

2 O=Chipping around hardness indentation-Rockwell A; N =No; significant chipping.

The results for nitrided titanium-base alloys containing colnmbium are similar to unalloyed titanium and the commercial titanium alloys; 20Cb-8OTi fails in the 400 s.f.m. test and shows pronounced spalling of the nitrided layer around a hardness impression. Nitrided Cb-60Ti and Cb-40Ti also fail the test criteria. Nitrided, unalloyed colnmbium also forms discreet surface nitrided layers which separate from the metal substrate upon testing and this material fails. Similar results were obtained for Cb-6Ti and these materials are excluded from our invention. We noted a modest improvement with nitrided Cb-l 0Ti which cut somewhat at 400 s.f.m. but it really was not too good as compared with the materials falling within the scope hereof and it is excluded from our invention. A significant difference in performance was observed with nitrided Cb-20Ti and Cb-30Ti; both materials passed the cutting and chipping propensity tests and are included within our invention.

Similar trends are observed for nitrided tantalumtitanium alloys. Nitrided Ta-70Ti fails at 400 s.f.m. and shows chipping. Unalloyed tantalum and Ta-Ti fail. Nitrided Ta-lOTi is a marginal material that did cut at 400 s.f.m. but after such testing chipping was observed at the specimen edges. A slight chip also occurred adjacent to the Ra indentation, and We do not include this material within our invention. On the other hand, the nitrided alloys Ta-20Ti and Ta-30Ti show excellent performance and are included within our invention. The Ta-40Ti material does not chip and removes 2 cubic inches of steel at 400 s.f.m., but the tool was severely worn. For comparison the nitrided Ta-30Ti tool exhibits a 0.005 inch uniform wear/ 0.010 inch localized nose wear after cutting at 400 s.f.m. and this material also cuts effectively at 750 s.f.m. The Ta-40Ti material is included within our invention, but this represents the highest titanium content of this invention. Nitrided Ta-SOTi fails the test criteria.

Another nitrided alloy system included herein is vanadium-titanium. Nitrided unalloyed vanadium, V-lOTi, V-50Ti, and V-60Ti all failed and are excluded from our invention. Nitrided V-20Ti passed our test criteria and falls within the scope hereof. Nitrided V-40Ti passed the 400 s.f.m. test, but it showed a chipping propensity and as a marginal composition we exclude it from our invention. A similar duplex nitriding treatment of 2000 F. plus 2450 F. for two (2) hours that resulted in some improvement for the V-20Ti was applied to the V-60Ti alloy but the latter still failed in cutting and chipped.

Nitriding of columbium-zirconium alloys yielded similar results. The nitrided zirconium-base compositions Cb-60Zr and Cb-SOZr fail in cutting at 400 s.f.m., show chipping, and are excluded herefrom, but good performance is found with nitrided Cb-20Zr which is part of our invention. With Ta-Zr alloys, the nitrided Ta-SOZr fails in cutting at 400 s.f.m., shows chipping, and is excluded. However, as shown in Table I, a range of tantalum-base alloys containing zirconium pass at 400 s.f.m. and show no chipping. Nitrided Ta-ZOZr and Ta-30Zr showed particularly good performance and cut effectively at 750 s.f.m. as well. For the nitrided V-Zr system, V-80Zr fails to pass and is excluded from our invention, but V-35Zr passes and is included.

In addition, we have combined titanium plus zirconium in our nitrided compositions and find useful materials are produced. For example, nitrided Cb-15Ti-l5Zr and Ta-l5Ti-15Zr both pass the 400 s.f.rn. and Ra chipping propensity tests. Nitrided ternary alloys are included within our invention that contain one metal of the group columbium, tantalum, and vanadium, and both metals of the group titanium and zirconium wherein the range of titanium and zirconium is 12 to 35% when columbium or vanadium are present and 12 to 40% when tantalum is present.

A variety of nitrogen containing environments can be used to produce similar hardened materials. However, upon reacting in a much lower nitrogen potential environment, the effect of lowered nitrogen availability is observed and a somewhat modified reaction product may be obtained. For example, similar size samples of Cb-20Ti were nitrided at 3050" F. for two hours and the weight pick-up was 20, 18, and 9' mg. with atmospheres that were nitrogen, argon-5% nitrogen, and argon-0.1% nitrogen, respectively.

Since our surface reacted composites are in a thermodynamically metastable condition, those skilled in the art will realize that a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction product and resulting properties whether performed as part of the over-all nitriding reaction or as separate treatments. The materials can also be nitrided at higher temperatures (and times) that normally would produce some embrittlement and then subsequently annealed in inert gas or various partial pressures of nitrogen as a tempering or drawing operation to improve toughness. This duplex treatment results in a deeper reaction product with the hardness-toughness relationship controlled by the tempering temperature and time.

In practicing'the teachings of our invention, from the foregoing it should be borne in mind that nitriding times and temperatures are variable over a considerable range. Generally, for cutting tool uses we nitride for around 2 hours at the temperatures shown as useful in Table I.

In addition to nitriding alone we have modified our nitrided material by combining nitriding wtih a very modest amount of oxidizing or boronizing. However, the amount of reaction with such other hardening agents must be limited, a majority of the weight pick-up is due to nitriding, and these are essentially nitrided materials. The alloys may be preoxidized at a temperature where little reaction would occur With nitrogen alone and then though the relative oxidizing potential must be low since for example in air the alloys will preferentially oxidize rath'er than nitride. A sample of Cb-15Ti-l5Zr Was nitrided at 3050" F. for 2 hours and subsequently boronized at 2650 F. for 4 hours. The structural features of such a material are very similar to the alloy only nitrided; the hardness grades inwardly and of the total weight pick-up over is due to nitriding. A smooth surface layer about 0.5 mil thick forms due to the boronizing treatment that is harder than the nitrided surface. Up to 25% of the nitrogen pick-up by weight may be replaced by oxygen and/ or boron.

The present useful alloys may be produced by powder processing techniques in addition to the treatment of solid metal stock as described above. Furthermore, such alloys may be employed on another metal or alloy as a surface coating or cladding and with the proper selection, a highly ductile or essentailly unreacted substrate can be thus obtained. For example, columbium or tantalum are much less reactive to nitrogen when used in conjunction with the alloys and molybdenum is essentially inert to nitrogen. Spraying and/or fusing the desired alloy onto the surface are included in the various coating methods available. Small other additions may be made to our alloys to enhance the coatability. A variety of direct deposition methods may be employed or alternate layers could be deposited followed by a diffusion annealing treatment. We have fused our reactive alloys on unalloyed molybdenum prior to nitriding and found good adherence and wetting. The nitrided material can be used as a mechanically locked insert or it can be bonded or joined by brazing, for example, to a substrate.

To illustrate the utility of surface alloying techniques for the preparation of the alloys of our invention as coatings, unalloyed columbium was titanized and nitrided, and microhardness traverse data are shown in Table II. The as-titanized specimen shows some small solid solution strengthening near the surface. After nitriding this specimen, the grading of hardness inwardly from the surface through the titanized zone which is about 5 mils deep is apparent. However, when unalloyed columbium is nitrided, there is formed a discrete nitrided layer(s) and the abrupt change in hardness can be seen in Table II at a depth of between 1 and 2 mils.

A bend specimen was prepared by titanizing a columbium strip 75 mils thick with the following procedure: vacuum pack titanized 2950 F.6 hours; annealed in argon 2950 F.-*2 hours+3050 F.-2 hours. After such treatment, the specimen was bent at 40 F. Cracking of the hard nitrided case occurred on the tension (outside) side only. None of the hard nitride Cb-Ti material spalled from the columbium substrate which was intact. An insert cutter of tantalum titanized and nitrided under the same conditions as the columbium bend specimen cut at 400 s.f.m. although the wear was relatively high.

TAB LE II Mlcrohardness (DPN) at distance from surface (mils) 1 Vacuum pack titanized 2.950 F.-3 hrs. Nitrided 3,250 F.-2 hrs.

The high surface hardness of the nitrided alloys has been measured by 50 and 200 gram diamond pyramid microhardness traverses on metallographically-polished cross-sections. For the 4; inch thick materials falling within our inveniton the hardness measured at about 0.5 mil from the surface is in the range of 1000 to 2500 DPN and the hardness grades inwardly in a mostly continuous fashion. The nitrogen pick-up is in excess of 1 mg. per sq. cm. for all of the examples shown in Table I. However, the amount of nitrogen required for an equivalent surface hardness is substantially reduced when the material is used as a thin blade edge or sheet or as a thin coating or cladding. Also, such materials may be used for a wide variety of applications requiring wear and abrasion resistance where the requirement for surface hardness or depth of hardening may be less than that required for metal cutting. Thus, for certain applica tions, the nitrogen pick-up might be 0.1 to 1 mg. per sq. cm. of surface area.

We have also observed the excellent corrosion resistance of both the alloys and the nitrided alloys in strong acids, and these materials could effectively be employed for applications requiring both corrosion and abrasion resistance. Both the alloys and the nitrided alloys possess good structural strength. Thus, the materials can be employed for applications involving wear resistance and structural properties (hardness, strength, stiffness, toughness) at room and elevated temperatures. Other useful properties of the nitrided materials include good electrical and thermal conductivity, high melting temperature, and thermal shock resistance.

Although the alloys receptive to nitriding can be produced by coating or surface alloying techniques, many uses involve the forming and machining of a homogeneous alloy. One of the advantages in utility of these materials is our ability to form the metallic alloys by cold or hot working and/or to machine (or hone) to shape in the relatively soft condition prior to final nitriding. Only minimal distortion occurs during nitriding and replication of the starting shape and surface finish is excellent. The final surface is reproducible and is controlled by original surface condition, alloy composition, and nitriding treatment. For some applications, the utility would be enhanced by lapping, polishing, or other finishing operations after nitriding. The nitrided surface is quite hard but only a small amount of material removal is required to produce a highly finished surface.

The excellent cutting properties and wear resistance of the nitrided materials can be effectively employed with the other useful properties of the alloys and nitrided materials to produce a wide range of products. Some of these are: single point cutting tools, multiple point cutting tools (including rotary burrs, files, routers and saws), drills, taps, punches, dies for extrusion, drawing, and other forming operations, armor, gun barrel liners, impeller or fan blades, EDM (Electrical Discharge Machining) electrodes, spinnerts, guides (thread, wire, and other), knives, razor blades, scrapers, slitters, shears, forming rolls, grinding media, pulverizing hammers and rolls, capstans, needles, gages (thread, plug, and ring), bearings and bushings, nozzles, cylinder liners, tire studs, pump parts, mechanical seals such as rotary seals and valve components, engine components, brake plates, screens, feed screws, sprockets and chains, specialized electrical contacts, fluid protection tubes, crucibles, molds and casting dies, and a variety of parts used in corrosionabrasion environments in the paper-making or petro chemical industries, for example.

It will be understood that various modifications and variations may be affected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. A graded, nitrided binary or ternary alloy material having excellent wear and abrasion resistant properties consisting essentially of tantalum and a metal selected from the group consisting of titanium and zirconium and mixtures thereof wherein (a) the nitrogen weight pick-up is at least 0.1 milligram per square centimeter (b) the content of a metal selected from the group titanium, zirconium and mixtures thereof ranges from 20% to 30%; and

(0) being characterized, when sufficiently nitrided and used as a cutting tool by the ability to remove at least 2 cubic inches of hardened 4340 steel, Rockwell C44, at a cutting speed of 750 surface feet per minute.

- References Cited UNITED STATES PATENTS 2,015,509 9/1935 Austin 14820.3 2,170,844 8/1939 Van Note 14820.3 3,038,798 6/1962 Berger et al. 75-177 X 3,161,503 12/1964 Lenning et a1. 75--l74 3,163,563 12/1964 Douglass et al. 75174 X 3,392,126 7/1968 Bindari 75l74 X OTHER REFERENCES IR 7187(III) 11T Research Report, June 16, 1967- Sept. 15, 1967, pp. 51-56, 58, 59, 60, and 67.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

